This invention relates to electronic radar in general and more particularly to weather radar having ground clutter identification capability.
Weather radar has been found to be extremely useful in locating thunderstorms and other forms of precipitation, thereby enabling improved accuracy in weather forecasts which allows pilots of aircraft to detour around these lightly hazardous regions.
The weather radar typically operates at relatively high frequencies since the precipitation, (raindrops, snow, hail) is composed of large numbers of relatively small radar targets and therefore the high frequency end of the radar spectrum has been found to be more effective in identifying these regions.
One significant problem observed with horizontally aimed or downward projecting weather radars has been the observance of "ground clutter" resulting from the radar reflections from stationary ground objects such as buildings, hills, etc. One approach to this problem has been the use of pulsed Doppler radar which can eliminate returns through electronic processing in the receiver from objects which are stationary relative to the broadcast station. The use of Doppler notch filters in this regard is described in U.S. Pat. No. 4,377,811 issued Mar. 22, 1983 entitled "Medium PRF Pulse Doppler Radar Having a Simplified Ground Moving Target Rejection Capabilities" to David H. Moonie, et.al. That technique is generally useful, however it is limited to stationary clutter and necessarily reduces the signal strength as a function of the filter capabilities and thereby reduces overall system performance. Additionally, when the weather radar is mounted on a moving platform, such as an aircraft, or when the ground targets additionally have a velocity component such as that caused by the wind disturbing leaves causing them to move relatively rapidly, these forms of ground clutter will not be effectively filtered by a Doppler notch filter.
It is known that the design techniques necessary for the detection of targets in a clutter background are considerably different from those techniques necessary for detection in a noise background. The text entitled "Introduction to Radar Systems" by Merrill I. Skolnik discusses these techniques in detail in Chapter 13, pp. 470-516, and is hereby incorporated by reference thereto.
Additionally, equipment limitations such as inherent receiver noise, radar transmitter stability, normal signal processing errors, and variances resulting from a wide variety of targets are potentially encounterable. Coupled with the problems associated with moving antennas, such as the angular movement between samples for a pulsed radar, and antenna beam width with the associated beam broadening, scan angle, and aircraft velocity, these problems provide significant hurdles in developing an aircraft mountable weather radar capable of rejecting moving ground clutter as well as compensating for aircraft velocity, while still providing an effective and useful representation of hazardous weather regions for the aircraft.